Circuit designers have traditionally used bandgap voltage reference circuits to generate reference voltages that do not vary significantly over an operating temperature range. The output of a bandgap voltage reference circuit is independent of the temperature at which the circuit is operating. For example in data acquisition systems, a voltage reference circuit typically generates a voltage based on a temperature stable bandgap reference since the accuracy of the voltage reference limits the gain accuracy of the system.
Traditional bandgap voltage reference circuits generate a difference in base-to-emitter voltages of two bipolar transistors, known as delta VBE or ΔVBE, which is proportional to absolute temperature (PTAT). The ΔVBE voltage is added to the base-to-emitter voltage of a single bipolar transistor, which is complementary to absolute temperature (CTAT), to form an output voltage. If the proportional variation with respect to temperature can be made to accurately offset the complementary variation with respect to temperature, then the resulting output voltage will be temperature compensated such that its variation will be small over the operating temperature range. While the output voltage of a bandgap voltage reference circuit is temperature compensated, it can have some variation over temperature due to several imperfections in the circuit. For example, a mismatch between the two transistors forming the ΔVBE voltage can cause some temperature variation. In addition to some temperature variation, bipolar transistors produce low frequency noise, and the bandgap reference circuit amplifies the noise.